A drug delivery system (DDS) for making an effect of drugs to exhibit at the maximum is essential in a pharmaceutical development field, and development of a carrier for delivering the drugs is important. A carrier system utilizing a polymer nano-assembly allows construction of multifunctional polymer devices by a precise molecular design of a polymer chain, and has been anticipated as a DDS carrier application. As a polymer carrier used in this field, micelles, nano-gel particles, nanospheres, nano-capsules, and the like are known, and spontaneous assembly of polymers due to inter- or intra-interaction of the polymer chains has been utilized as driving force for particle formation in preparation of these nano-carriers. Examples of interaction worked in this spontaneous assembly include hydrophobic interaction, electrostatic interaction, hydrogen-bonding, van der Waals force, and the like (Patent Documents 1-13).
In particular, a polyion complex (hereinafter, abbreviated as “PIC”) which comprises polymers having opposite charge (cations and anions) and a PIC nanoparticle having a particle shape have an advantage that they can be easily prepared only by mixing two kinds of polymers in an arbitral ratio and, therefore, various PICs and PIC nanoparticles utilizing synthetic or naturally occurring polymers have been reported (for example, Patent Document 14). However, because formation of these PICs and PIC nanoparticles utilizes Coulomb's force as driving force, there has been problems that stability of the nanoparticles is poor against a high salt concentration such as in buffers and pH alteration and, therefore, a use thereof under physiological environments is limited.    [Patent Document 1] Japanese Patent Publication No. 2000-5296A    [Patent Document 2] Japanese Patent Publication No. 2000-126585A    [Patent Document 3] Japanese Patent Publication No. 2001-81237A    [Patent Document 4] Japanese. Patent Publication No. 2001-146556A    [Patent Document 5] Japanese Patent Publication No. 2002-638A    [Patent Document 6] Japanese Patent Publication No. 2004-294231A    [Patent Document 7] Japanese Patent Publication No. 2004-352972A    [Patent Document 8] Japanese Patent Publication No. 2005-8614A    [Patent Document 9] Japanese Patent Publication No. 2006-67889A    [Patent Document 10] Japanese Patent Publication No. H05-84665A    [Patent Document 11] WO 06/25419    [Patent Document 12] WO 06/90924    [Patent Document 13] WO 08/62909    [Patent Document 14] Japanese Patent Publication No. H10-77342A